Coated board of wood-based material

ABSTRACT

The present application relates to a coated board of wood-based material and a method for coating a board of wood-based material, wherein the board of wood-based material is in particular a wall panel or floor panel or is intended for producing such a panel, comprising a front side and a rear side, wherein at least the surface of the front side is provided with a polymer coating and wherein the polymer coating has a hardness gradient, so that the hardness of the polymer layer decreases with increasing depth from the surface.

1. FIELD OF THE INVENTION

The present invention relates to a coated board of wood-based material,in particular for producing a floor, ceiling or wall covering as well asa method for coating a board of wood-based material.

2. BACKGROUND

A plurality of covering boards on wood-based material are known from theprior art. In the simplest case such a board consists of a solid realwood. Such boards of solid wood are however very expensive and as panelsit only can be laid by well skilled specialists. However, such so-calledreal wood planks provide a highly attractive surface. In order to avoidhigh costs of real wood floorings and to provide the attractive surfaceof such floorings at the same time, veneer covering boards have beendeveloped. Veneer are thin sheets, as a rule 0.3 to 0.8 mm, from a highquality wood which are applied with glue to a base material. As a rule,the base materials consist of cheaper wood-based materials and arestrikingly thicker than the veneer layer. A drawback of such coveringsis the relative sensitive surface which, for example, can be easilydamaged by means of wetness or by means of mechanical action.

Furthermore, laminate panels for floor or ceiling coverings are knownfrom the prior art. In comparison with the covering boards mentioned atthe beginning, laminate panels are relative inexpensive. As a rule,laminate panel consists of a 4 to 12 mm thick base board of MDF or HDFraw material thus of a relative low priced wood-based material whereinonto its upper side a paper printed with a décor is bonded. As a rule,at the bottom side of the base board there is situated a so-calledcounteracting paper which is to counteract a distortion of the baseboard by means of the applied décor layer. In order to improve thedurability of the dé-cor layer, a so-called overlay paper is typicallyapplied onto the décor layer wherein the overlay paper is impregnatedwith a resin, for example an amino resin, and onto the resin are appliedvery fine abrasion-resistant particles as for example aluminium oxideparticles. By pressing under application of heat and pressure thedifferent layers of the laminate panel are joint together and the usedresins are cured. Therefore, the result is a durable abrasion-resistantdecorative surface.

In order to improve the durability and thus also the optical propertiesof the boards of wood-based material, as they are used for example forwall, ceiling or floor panel, there have been recommended severalmethods for coating and materials in the prior art. In principle suchcoatings can be applied onto any kind of board of wood-based material,including the above mentioned real wood panels and laminate panels, inorder to increase the durability of the surfaces.

For example, a method for coating of a board of wood-based material isknown from the WO 2007/042258 A1, wherein in a single coating step arelative thick protective layer of plastic material is applied onto thesurface of a board. The used plastic material thereby is apolymerisation able acrylate system which can cure by means of apolymerisation. The polymerisation is started by means of radiation sothat a complete conversion occurs through the thickness of the appliedlayer.

Based from these prior art there is the object to provide a coated boardof wood-based material and also a method for coating a board comprisingspecific advantageous mechanical properties.

These and other objects will be apparent in the following description orwill be recognizable from the person skilled in the art and will besolved with a coated board of wood-based material according to claim 1and with a method for coating according to claim 9.

By means of the present invention abrasion values of the highestabrasion grade AC 5 according to prEN 15468 are achieved by optical goodtransparency of the coating and furthermore by good brilliance of aprinted design applied underneath or therein. The surface ischaracterised by high micro scratch resistant (Mar-Resistance) andimpact resistance according to grade 33 (prEN 15468). The characteristicvalues for chemical resistance and water vapor resistance, castor chairtest and case leg test are certain in accordance with the prEN 15468.Furthermore, the method allows a surface in which additionally to thepressure a deep embossed decorative structure for example a brushed woodstructure or a stone structure can be brought in. The invention istherefore particularly suitable for providing of floor panels.

3. DETAILED DESCRIPTION OF THE INVENTION

The coated board of wood-based material is in particular a floor,ceiling or wall panel and respectively a board of wood-based materialwhich is provided for further processing to a floor, ceiling or wallpanel, and comprises a front side and a rear side wherein at least thesurface of the front side is provided with a polymer coating. The termboard of wood-based material is to understand wide and comprises forexample both boards made of real wood and boards made of MDF, HDF, chipboards, composite boards, OSB boards and the like. The board ofwood-based material can further be provided with additional coatings,papers, veneers or the like onto their surfaces of front side and/orrear side. Thus, when a coating of the surface of the board ofwood-based material is mentioned, this necessarily means not a directcoating of the board of wood-based material, but the same for examplecan be provided with a décor paper, wherein the coating is then appliedonto the décor paper. According to the invention the polymer coatingcomprises a hardness gradient after curing so that the hardness of thepolymer layer decreases with increasing depth viewed from the surface.That is, the polymer layer has preferably the maximum hardness at itsouter surface and has the minimum hardness nearby the boundary surfacebetween coating and surface of the board of wood-based material, with adecreasing course between the both extremes.

Up to now it has always been desired to achieve preferably a maximumhardness over the over-all layer thickness. The coating according to theinvention deviates from this teaching and however surprisingly resultsin excellent mechanical durability values. An explanation thereforecould be that by means of a preferably steady decrease of hardness therenot occur high peaks in the properties of the coating and therefore thecoating is particularly durable.

The present invention also relates to a method for coating a board ofwood-based material, in particular a floor, ceiling or wall panel, andrespectively to a board of wood-based material which is processed to afloor panel, wherein in a first step a first liquid coating means isapplied onto a board of wood-based material and onto the still wet firstcoating means a second liquid coating means is applied, wherein theliquid layers penetrate each other according to the physics of liquids.The outcome of this is a gradient of the concentration of both liquids.While in the outer areas of the total layer (upper side respectivelylower side of the over-all layer) the respective liquid of the originalsingle layers is pre-dominant, there exists a concentration gradient ofthe first liquid and respectively of the second liquid to the centre andalong to the respective other side of the layer. In the ideal case therespective gradient course corresponds to a straight line. Since in caseof higher viscous liquids at short mixing times interruptions may occurto the ideal case, one has to assume that the effective concentrationcurves only approximately correspond to straight lines and deviationsare possible. When the liquids for example are polymerisation ableacrylate systems, which are different in the double pond rate, so itfollows from the above mentioned that analog to the concentrationgradient of the both liquids together, a gradient arises in the numberof the double bonds from one side to the other side of the layer. Whennow a polymerisation is actuated in such a layer, for example by meansof UV radiation, and one assume that under inert conditions an almostcomplete conversion of the double bonds occurs so there arise a polymerlayer with a gradient of the cross-linking points. While the side withhigh double bond concentration is accordingly strong cross-linked, theother side with the low double bond rate has accordingly a lowercross-linking. According to the polymer physics the hardness of such asystem gives an information of the cross-linking density. When, forexample, the micro hardness (Martens hardness DIN EN ISO 14577) ismeasured within a layer which is accordingly produced from twopolymerisation able liquids, there occurs a hardness gradient analog tothe cross-linking density. The layer can be removed in stages forexample with a Taber-Abrasion-Test (Taber-Abraser-Test) according to EN13329. The curve progression of the hardness gradient similarlycorresponds to the above described concentration gradient of bothliquids. In the ideal case of the mixing of the liquids straight linesoccur. In practice, however, there will occur deviations to the straightlines. Mathematical it may therefore be expected that the functiony=f(x) has a progression deviating from a straight line (wherein y isthe Martens hardness and x is the abrasion depth in the layer).

The described context shall be illustrated to the person skilled in theart with the following example:

Onto a HDF base board a first layer of 45 g/m² is rolled on via a rollapplicator wherein the coating means of the first layer for exampleconsists of 35% from a 1, 6 hexanediol diacrylate and of 65% from apolyester acrylate. A second layer with a mass of 40 g/m² is immediatelyapplied thereafter onto this layer wherein the coating means of thesecond layer for example consists of a mixture of 70% polyurethaneacrylic ester and of 30% dipropylene glycol diacrylate. Both layerspresently include a photoinitiator. The so produced liquid over-alllayer is subjected to a UV radiation under nitrogen atmosphere and theover-all layer is polymerized. The double bond conversion thereby isapproximately 98%.

In order to analyze the resultant coating, the coating has subsequentlygradually been removed with the Taber-Abraser-Test by means ofrespectively 200 rotations (described in the EN 13329). The Martenshardness was respectively measured of each an abrasion step. When onechart in a coordinate system the Martens hardness in N/mm² to the y-axisand the corresponding abrasion depth in μm to the x-axis, the outcome ofthis is approximately a straight line with the function y=134.8-1.03×.The coefficient of determination has been determined with 87.8% whichshows a very high accuracy of this mathematical correlation forwood-based materials.

When coatings according to the invention for example are used for ahard-wearing floor covering the layers may additionally be provided withabrasion-resistant particles, such as fine corundum particles. Theseparticles may for example be present in one or both coating means in adispersion before the coating process or the particles can be spreadonto the still wet but already applied coating means in a separateprocess step.

The person skilled in the art recognizes on the basis of the presentdescription of the invention that according to the application coatingmeans can be used with other concentrations as preferably denoted in theexample. Preferably the concentration of 1, 6 hexanediol diacrylate canbe between 10 and 60%, more preferably between 20 and 40%; theconcentration of polyester acrylate can be between 40 and 90%, morepreferably between 50 and 80%; the concentration of polyurethane acrylicester can be between 45 and 95%, more preferably between 55 and 75% andthe concentration of dipropylen glycol diacrylate can be between 5 and55%, more preferably between 15 and 35%. The mentioned substances shallclarify the principle of layer with hardness gradients according to apreferred embodiment. It is self-evident that a plurality of further orother polymerisation able substances can be used instead of the abovementioned. Polymerisation able acrylates are particularly preferredsubstances for the herein described coatings.

The coating means of the first layer as well as of the second layer andmaybe of further layers can consist of a single polymerise ablesubstance or of mixtures of substances. Particularly preferred suitablesubstances are polymerising able acrylates as in general and here inparticular the substances: 1, 6 hexanediol diacrylate, polyesteracrylate, polyurethane acrylic ester and dipropylen glycol diacrylate.Particularly suitable for the first layer is a mixture of 1, 6hexanediol diacrylate and polyester acrylate. For the second layer is amixture of polyurethane acrylic ester and dipropylen glycol diacrylateparticularly suitable.

In the coatings means further additives can be present such as flowadditives, wetting additives, dyestuffs, abrasion-resistant particlesand so on. Important therefore is that these further components allowthe above described cross-linking and penetration, respectively, andthat a polymerisation is still possible.

By selecting of the coating means for the single layer the mentionedsubstances are preferred, however, the person skilled in the artrecognizes that it does not depend on the use of the denoted substancesbut substantially on the provision of polymerise able coating means.

4. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following, a detailed description of exemplary embodiments willbe given by means of the enclosed diagrams and figures.

FIG. 1 is a schematic illustration of a coating process;

FIG. 2A to 2C are schematic illustrations in which the procedure ofmixing of two liquid layers is shown;

FIG. 3 is a diagram, which shows the course of the hardness against thedepth of the coating;

FIG. 4 is a diagram, which illustrates the upper and lower boundaries ofthe hardness gradient according to a preferred embodiment of theinvention;

FIG. 5 is a diagram, which illustrates the upper and lower boundaries ofa to more preferred embodiment of the invention; and

FIG. 6 is a diagram, which illustrates the upper and lower boundaries ofthe hardness gradient of a further preferred embodiment.

In FIG. 1 a coating plant for coating of boards of wood-based material10 is schematically shown. The boards of wood-based material 10, such asboards of solid wood, HDF, MDF or chip boards, are guided by means of aroller conveyer plant 12 through the different stations of the coatingplant. In a first coating station 14 a first liquid coating means 20 isapplied in a passage coating onto the boards of wood-based material 10by means of a rotating applicator roller 15.

The applicator roller 15 is provided with coating means by means of asupply device 16. In the second coating station 17 a second liquidcoating means 21 is applied onto the still wet first coating means 20 bymeans of a further rotating applicator roller 18. The applicator roller18 is provided with the second liquid coating means by means of a supplydevice 19. It is self-evident that the applying can also be done withany other suitable applying process, such as by means of a sprayingdevice or a coating blade or the like. Therefore, it is only importantthat the applying of the second layer takes place as long as the firstlayer is still wet enough, so that a partial mixing of the two layerscan take place. Furthermore, it is self-evident that further coatingstations can be provided after the second coating station 17 in order toapply for example a third liquid coating means onto the still wet secondcoating means 21 or also additional stations in order to applyabrasion-resistant particles onto and respectively into the wet layers.

After leaving of the coating station 17 the coated boards 10 areconveyed to a hardening station 30, where the layers are hardened bymeans of UV radiators 31. On their way from the coating station 17 tothe hardening station 30 a partial mixing of the liquid coating means 20and 21 occurs, which particularly takes place at the boundary surfacesof the two coating means. Thereby, naturally the mixing is stronger, thecloser one is located at the boundary surface of the two layers. Bycuring of the layers in the curing station 30 the mixing process isstopped and the once adjusted mixing proportion and therefore themechanical properties of the produced coating is set. The extent of themixing at the boundary surfaces—which takes place itself and preferablywithout external mechanical action—depends on the time duration whichpasses between the applying of the second coating means 21 onto thestill wet first coating means 20 and the curing in the curing station30. Furthermore, the mixing of the two coating means is also influencedby the respective viscosity of the coating means wherein the generalrule is that the higher the viscosity, the lower the mixing per timeunit.

The principle of the mixing of the two applied coating means can be seenbest from the schematically illustration of FIG. 2A to 2C. Therefore,FIG. 2A shows the condition of the two coating means 20 and 21 appliedonto a board of wood-based material 10 immediately after applying of thesecond coating means 21. At that time practically no mixing has takenplace. In the present case, the coating means 20 and 21 are polymers,which have respectively different numbers of C—C carbon double bonds.Therefore, as schematically depicted in FIG. 2A, the first coating means20 has a lower number of C—C double bonds than the second coating means21. Due to the higher number of C—C double bonds in the coating means21, the same will have a higher hardness after the curing than thecoating means 20 which is provided with lower amount of C—C doublebonds.

As the two coating means 20 and 21 are applied wet on wet, a mixing ofthe two layers occurs starting from the boundary surface 22 of the twolayers, as it is indicated in FIG. 2B. This means that due to the mixingprocess in the area close to the boundary surface 22 there are moredouble bonds in the underlying layer and accordingly in the area closeto the boundary surface 22 of the overlying layer there are fewer doublebonds, as before the mixing. FIG. 2C shows the two layers after themixing has advanced some more and has reached a suitable mixing grade.If at this point of time the curing of the coating means occurs, forexample by means of UV radiation, this mixing rate is set, since in thehardened layers naturally no mixing can occur any more.

In the diagram of FIG. 3 the hardness course of a coating according tothe invention (example with hardness gradient) and a coating accordingto the prior art are plotted. The example according to the inventionconsisted of an abraded board of wood-based material provided with aprimer on which the two different coating means were applied wet on wet.The first applied coating means consisted of approximately 35% 1, 6hexanediol diacrylate and approximately 65% polyester acrylate and wasapplied with 45 g/m². The second coating means which was applied ontothe still wet first layer consisted of approximately 70% polyurethaneacrylic ester and approximately 30% dipropylene glycol diacrylate andwas applied with 40 g/m². After applying of the second layer there was awaiting time of 10 seconds in order to make it possible for the viscousliquid materials to mix. Afterwards, the two layers were completelyhardened together.

The example according to the state of the prior art consisted of aconventional coating, wherein multiple thin layers of materials wereapplied separately and wherein between the respective applyingprocedures the pre-applied layer was hardened. The lower three layersconsisted of a mixture of 70% polyester acrylate and 30% 1,6 hexanedioldiacrylate with an applying intensity of 12 g/m². The two upper layersconsisted of 70% polyurethane glycol diacrylate and 30% dipropyleneacrylic ester and the two upper layers contained 15% corundum with anaverage particle size of D 50 of 25 μm.

The test was carried out according to the European standard for laminatepanels DIN EN 13329 with a Taber-Abraser-Tester 5151 of TaberIndustries. After 200 rotations respectively with S-41 abrasive paperthe hardness and the trace depth of the samples were determined. Thedetermination of the Martens hardness (registering hardness test undertest application of a force) was carried out according to DIN EN ISO14577. A “Fischerscope H100” of Helmut Fischer GmbH was used as a testapparatus. The following test parameters were used: maximal strength:50/30 mN as well as measuring period: 20 seconds. The determination ofthe trace depth was carried out with a mechanic brush analyzer. APerthometer S3P of Perthen was used as a test apparatus.

During the measurement of the samples it became apparent that probablydue to the used relative soft materials more or less deviations occur inthe hardness of a given layer depth. Therefore, it is necessary tomeasure at several points in order to get representative data by meansof an average determination. During the carried out measurements thehardness as well as the trace depth was respectively measured after 200rotations of the abrasive paper at four points. It became apparent thatin most of the majority of cases four measurement points provide asufficient accuracy. It is self-evident that one can get more accuratemeasurement results by using more than four measuring points, like eightfor example.

In the below depicted table the individual measured data for the sampleof the example according to the invention are depicted. The measurementwas carried out on the completely cured coating that means the conditionin which respective products would be really used as floor panel.

TABLE 1 Example with hardness gradient depth measurement depth trace[μm] of hardness [μm] Martens hardness [N/mm²] rotation 1 2 3 4 1 2 3 41 2 3 4 3.6 3.8 3.3 3.4 134.8 118.7 159.0 150.6 AV 3.5 140.8 200 20.020.0 20.0 20.0 3.5 3.7 4.3 3.9 139.7 125.2 93.5 112.2 AV 20.0 3.9 117.7400 20.0 20.0 20.0 25.0 4.5 5.0 4.0 3.9 85.9 69.9 108.9 113.2 AV 21.34.4 84.5 600 25.0 25.0 25.0 30.0 4.7 4.7 4.3 4.0 80.5 79.6 95.0 106.1 AV26.3 4.4 90.3 800 30.0 30.0 30.0 35.0 4.1 4.1 4.0 4.2 103.8 103.1 109.7100.3 AV 31.3 4.1 104.2 1000 40.0 40.0 40.0 45.0 4.7 4.2 3.9 4.5 78.599.3 112.0 87.5 AV 41.3 4.3 94.3 1200 50.0 50.0 50.0 50.0 4.3 5.4 4.24.6 93.7 59.8 98.5 82.6 AV 50.0 4.6 83.7 1400 55.0 55.0 60.0 60.0 5.44.5 4.0 5.0 60.1 85.0 106.7 70.6 AV 57.5 4.7 80.7 1600 60.0 65.0 70.070.0 4.7 4.4 4.3 4.6 47.8 53.6 55.5 48.9 AV 66.3 4.5 51.5 1800 65.0 70.075.0 75.0 4.0 4.5 4.9 5.3 64.5 50.1 43.7 37.1 AV 71.3 4.7 48.9 2000 75.080.0 80.0 75.0 5.8 4.9 6.2 6.0 31.3 43.6 27.3 41.6 AV 77.5 5.5 36.0 220095.0 105.0 105.0 100.0 4.6 5.1 6.1 4.9 51.4 40.8 28.1 43.7 AV 101.3 5.241.0In the above depicted table the column “rotation” indicates the numberof rotations which were carried out with the Taber-Abraser-Tester. Thecolumn “depth trace” indicates how many micrometer material of thecoating starting from the original surface was removed at the fourmeasuring points 1-4. The column “depth measurement of hardness”indicates how many micrometers the test pin entered into the coating atthe four measuring points 1-4 respectively. In the column “Martenshardness” the hardness is indicated in Newton per mm² for the fourmeasuring points 1-4 respectively. Below the individual values therespective average value for the four measuring points is indicated.From the above depicted table it is easy to recognize that the Martenshardness decreases the deeper one penetrate into the completely curedlayer. It is also apparent that at 800 and 1000 (over-all) rotations amoderate rise of the Martens hardness can be noted. This is due to theirregular mixing of the two used coating means which in the praxis canonly fully be avoided.

Nevertheless it is apparent in the diagram of FIG. 3 that in the examplewith hardness gradient there is a nearly continuous decrease of hardnesswithout great peaks. However, the comparison example according to thestate of the prior art does not show such a continuous progress of thehardness, but moreover at a depth of 60 to 80 μm it has a pronouncedpoint of discontinuity up to the original initial hardness.

The average values of the test sample are depicted in thebelow-mentioned table 2.

TABLE 2 Average values of the example with hardness gradient depthMartens hard- Standard deviation of the rotation [μm] ness [N/mm²]Martens hardness [N/mm²] 3.5 140.8 15.4 200 23.9 117.7 17.0 400 25.694.5 17.6 600 30.7 90.3 11.0 800 42.1 104.2 3.4 1000 45.8 87.5 12.6 120054.6 82.8 14.9 1400 62.2 80.7 17.4 1600 70.8 51.4 3.2 1800 76.0 48.910.1 2000 83.0 35.9 6.8 2200 106.4 41.0 8.4

The values of the comparison test sample according to the prior art areshown in the below-mentioned tables 3 and 4.

TABLE 3 Sample according to prior art depth measurement depth trace [μm]of hardness [μm] Martens hardness [N/mm²] rotation 1 2 3 4 1 2 3 4 1 2 34 3.1 3.5 3.1 3.0 180.6 141.8 173.1 192.4 AV 3.2 172.0 200 30.0 25.025.0 25.0 4.2 4.2 3.7 4.7 99.9 99.6 124.5 79.3 AV 26.3 4.2 100.8 40035.0 35.0 35.0 35.0 3.7 3.8 4.0 4.1 126.9 117.2 110.1 105.3 AV 36.0 3.9114.9 600 45.0 45.0 45.0 45.0 3.7 3.8 4.6 4.8 128.4 122.2 83.2 74.7 AV45.0 4.2 102.1 800 50.0 50.0 50.0 50.0 4.0 4.7 4.8 4.0 108.2 80.8 75.4110.9 AV 50.0 4.4 93.8 1000 60.0 60.0 60.0 60.0 3.5 3.1 4.0 3.6 143.7177.4 108.0 129.9 AV 60.0 3.6 139.8 1200 66.0 70.0 70.0 70.0 3.3 3.4 3.63.0 160.7 145.1 135.0 185.1 AV 68.8 3.3 156.5 1400 70.0 75.0 75.0 75.03.3 3.0 3.1 3.8 157.7 191.6 178.0 119.3 AV 73.8 3.3 181.7 1600 76.0 80.080.0 80.0 2.3 2.9 2.6 2.4 183.8 124.8 147.9 174.4 AV 78.8 2.6 157.7 180080.0 85.0 85.0 85.0 3.8 3.0 3.4 3.1 71.4 112.3 88.6 107.0 AV 83.8 3.394.8 2000 85.0 90.0 85.0 85.0 5.1 3.5 2.6 3.0 40.9 82.3 146.4 112.6 AV86.3 3.6 95.6 2200 85.0 95.0 90.0 90.0 3.6 3.0 3.0 2.7 81.2 116.0 114.5137.5 AV 90.0 3.1 112.3 2400 90.0 100.0 100.0 95.0 3.7 5.2 3.1 3.0 77.639.7 108.2 111.8 AV 96.3 3.8 84.3 2600 100.0 100.0 105.0 100.0 5.3 3.36.0 3.9 37.8 92.6 42.4 67.7 AV 101.3 4.4 60.1

TABLE 4 Average values of the sample according to the prior art depthMartens hard- Standard deviation of the rotation [μm] ness [N/mm²]Martens hardness [N/mm²] 3.2 172.0 18.7 200 30.4 100.8 16.0 400 38.9114.9 8.1 600 49.2 102.1 23.5 800 54.4 93.8 15.9 1000 63.6 139.8 25.21200 72.1 158.5 18.9 1400 77.1 169.7 27.3 1600 81.3 157.7 23.1 1800 87.194.8 16.1 2000 89.8 95.6 38.9 2200 93.1 112.3 20.1 2400 100.0 84.3 29.02600 105.7 60.1 21.9

It has turned out experimentally that especially good mechanicalproperties of the complete over-all layer can be achieved, if thehardness gradient of the finished over-all layer—like it is shown in anexemplary manner in FIG. 3—esentially corresponds to the followingformula:

(−3.0*x)+C≦Y(x)≦(−0.2*x)+C

-   -   wherein:    -   x is the absolute value of the depth in μm of the coating viewed        from the surface of the coating;    -   Y(x) is the absolute value of the hardness in N/mm² at a certain        depth x; and    -   C is the absolute value of the initial hardness in N/mm² of the        coating at a depth of approximately x≈0-5 μm.

Under the “absolute” values it is to be understood that in the aboveformula only the plain numerical value is entered that means without theassociated measuring unit “μm” and “N/mm²” respectively. If, forexample, the initial value of the above example with hardness gradientis 140.8 N/mm² (see table 2), in the above table are inserted only theabsolute values, that means C=140.8. In the same way for x is insertedonly the absolute values, for example x=3.5. The result of this is, forexample, upper and lower boundaries for Y(x=3.5) of 140.1 and 130.3respectively. At a depth of x=40 μm the result is then, for example,132.8 for the upper boundary and 20.8 for the lower boundaryrespectively. These upper and lower boundaries for Y(x) have themeasurement unit N/mm². Important is that the absolute values, startingfrom the mentioned measurement units “μm” and “N/mm²”, are used in theformula and not starting, for example, from “mm” or “N/m²”. It should beclear for the person skilled in the art that the above formula is nomathematical formula to the description of the hardness gradient itself,but it rather defines a range, in which it should run.

The initial value of hardness of the coating is the value in the firstfew μm of the coating. Due to the typically used measurement method bymeans of a test pin which penetrates a few μm into the coating, it isdifficulty to determine the hardness for the depth of penetration “0μm”. The formulation “substantially” is therefore elected because it isdifficulty to achieve a perfect uniform mixing of the materials so thatin reality it can always come to single tiny outliers, such as thehardness value of 104.2 Newton/mm² at a depth of 42.1 μm (see table 2)of the above discussed example with hardness gradient. Furthermore, thevalues very close to the surface of the board of wood-based material aregenerally inaccurate, since the residual layer thickness to be measuredmust have a certain minimum thickness in order to allow usefulmeasurements. The residual layer thickness for useful measurementsshould therefore be at least 5 μm, preferably 10 μM and furtherpreferably at least 20 μm. With other words, the last 20 μm of thelayer, close to the board of wood-based material, must not necessarilyfollow the above mentioned preferred hardness gradient although this isnaturally preferred.

In a further preferred embodiment the hardness gradient substantiallyfollows the following formula:

(−2.5*x)+C≦Y(x)≦(−0.4*x)+C

And in another further preferred embodiment it substantially follows:

(−2.0*x)+C≦Y(x)≦(−0.6*x)+C

In the FIGS. 4 to 6 the meaning of the above mentioned formulas ofhardness gradients are illustrated according to examples with hardnessgradient. It should be clear that the indicated absolute values forhardness and depth are only exemplarily. It is self-evident that it ispossible to apply over-all layers with significant larger thicknesses orlower thicknesses. Furthermore, the absolute value of hardness certainlydepends on the used materials and can also be larger or lesser than thevalues of the example with hardness gradient. However, the order ofmagnitude of the cited values for the example with hardness gradient ismost preferred and suitable for the use in a floor panel.

The person skilled in the art recognizes by means of the detaileddescription of the method according to the invention how he can achievea coating of a board of wood-based material according to the invention.This means naturally that all materials mentioned and named inconnection with the description of the methods, such as the substancesfor the coating means, can also be used by the coating of the board ofwood-based material according to the invention.

The presented method is in particular suitable for coating of floorpanels, and respectively for coating of boards of wood-based materialswhich are subsequently further to floor panels processed since theadvantageously mechanical properties of the hardness gradient have herea strong effect. In the same way the presented coated board ofwood-based material is for the same reason preferably a floor panel andrespectively a coated board of wood-based material, which is intended tobe further processed to a floor panel.

1. Coated board of wood-based material, in particular a wall, ceiling orfloor panel, comprising a front side and a rear side, wherein at leastthe surface of the front side is provided with a polymer coating,characterised in that the polymer coating has a hardness gradient, sothat the hardness of the polymer coating substantially continuouslydecreases with increasing depth viewed from the surface of the coating.2. Coated board of wood-based material according to claim 1,characterized in that the hardness gradient substantially corresponds tothe following formula:(−3.0*x)+C<=Y(x)<=(−0.2*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 3. Coated board of wood-basedmaterial according to claim 1, characterized in that the hardnessgradient substantially corresponds to the following formula:(−2.5*x)+C<=Y(x)<=(−0.4*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 4. Coated board of wood-basedmaterial according to claim 1, characterized in that the hardnessgradient substantially corresponds to the following formula:(−2.0*x)+C<=Y(x)<=(−0.6*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 5. Coated board of wood-basedmaterial according to claim 1, characterised in that the board is a chipboard, MDF board, HDF board, OSB board or real wood board.
 6. Coatedboard of wood-based material according to claim 1, characterised in thatthe polymer coating consists of polymers which are curable by means ofradiation.
 7. Coated board of wood-based material according to claim 1,characterised in that the polymer coating has a initial Martens hardnessat a depth of approximately 0-5 μm from 120 N/mm² to 250 N/mm² measuredaccording to DIN ISO
 14577. 8. Coated board of wood-based materialaccording to claim 1, characterised in that the polymer coating has ainitial Martens hardness at a depth of approximately 0-5 μm from 130N/mm² to 200 N/mm² measured according to DIN ISO
 14577. 9. Method forcoating a board of wood-based material, comprising the following steps:a) providing a board of wood-based material; b) applying a first liquidcoating means; c) applying at least a second liquid coating means ontothe still wet first coating means, so that a partial mixture of thecoating means take place; d) curing of the applied coating means bymeans of radiation wherein the coating means are selected so that thecured resultant coating has a hardness gradient wherein the hardness ofthe coating decreases with increasing depth viewed from the surface ofthe resultant coating.
 10. Method for coating a board of wood-basedmaterial according to claim 9, characterised in that prior to step d)further coating means are applied onto the still wet pre-applied coatingmeans.
 11. Method for coating a board of wood-based material accordingto claim 9, characterised in that the hardness gradient substantiallycorresponds to the following formula:(−3.0*x)+C<=Y(x)<=(−0.2*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 12. Method for coating a board ofwood-based material according to claim 9, characterised in that thehardness gradient substantially corresponds to the following formula:(−2.5*x)+C<=Y(x)<=(−0.4*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 13. Method for coating a board ofwood-based material according to claim 9, characterised in that thehardness gradient substantially corresponds to the following formula:(−2.0*x)+C<=Y(x)<=(−0.6*x)+C wherein: x is the absolute value of thedepth in μm of the coating viewed from the surface of the coating; Y(x)is the absolute value of the hardness in N/mm² at a certain depth x; andC is the absolute value of the initial hardness in N/mm² of the coatingat a depth of approximately x≈0-5 μm.
 14. Method for coating a board ofwood-based material according to claim 9, characterised in that thefirst and the second layers are polymer layers, wherein the secondpolymer layer comprises more C—C double bonds than the first polymerlayer.
 15. Board of wood-based material coated by means of a methodaccording to claim
 9. 16. Using a coated board of wood-based materialaccording to claim 1 as wall, ceiling or floor panel.